The present invention is directed generally to radio communication systems and, more particularly, to techniques and structures for determining an appropriate power level for a base station to begin transmitting information to a mobile station on a traffic channel.
Traditionally, radio communication systems have employed either Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to allocate access to available radio spectrum. Both methods attempt to ensure that no two potentially interfering signals occupy the same frequency at the same time. For example, FDMA assigns different signals to different frequencies. TDMA assigns different signals to different timeslots on the same frequencies. However, neither methodologies completely eliminate interference. For example, TDMA methods allow for several users on the same frequency while avoiding interference caused by transmissions on adjacent timeslots of the same frequency through the use of synchronization circuitry which gates the reception of information to prescribed time intervals. Co-channel interference (i.e, interference arising from the re-use of the same frequency in different cells) is minimized to tolerable levels by only reusing frequencies in cells which are spaced a predetermined distance apart (the "reuse" distance).
In contrast, Code Division Multiple Access (CDMA) systems explicitly allow interfering signals to share the same frequency at the same time. More specifically, CDMA systems "spread" signals across a common communication channel by multiplying each signal with a unique spreading code sequence. The signals are then scrambled and transmitted on the common channel in overlapping fashion as a composite signal. Each mobile receiver correlates the composite signal with a respective unique despreading code sequence, and thereby extracts the signal addressed to it.
Regardless of the access methodology employed, at least some signals which are not addressed to a particular mobile station assume the role of interference. In CDMA systems, all signals interfere with one another due to the complete sharing of bandwidth, while in TDMA and FDMA systems, co-channel and adjacent channel interference arises from some signalling. To achieve reliable reception of a signal, the ratio of the received, desired signal to the interference should be above a prescribed threshold for each mobile station (referred to as a "required signal-to-interference" level, or SIR.sub.req). For example, as shown in FIG. 1, consider the case where three mobile stations receive, respectively, three signals from a common CDMA communication band. Each of the signals has a corresponding energy associated therewith--namely energy levels E1, E2 and E3, respectively. Also, present on the communication band is a certain level of noise (N). For the first mobile station to properly receive its intended signal, the ratio between E1 and the aggregate levels of E2, E3 and N must be above the first mobile's required signal-to-interference ratio.
Those skilled in the art will appreciate that similar situations exist for mobile stations in FDMA and TDMA systems when considering the effect of mobiles receiving signals on co-channels or adjacent channels. This situation can be conceptualized as illustrated in FIG. 2. Therein, base station BS1 is transmitting to mobile station 200 over a traffic channel using a particular frequency. At the same time base station BS2 is transmitting to mobile station 220 on the same frequency. Thus, any signal rays received by mobile station 220 from base station BS1 constitute interference which reduces mobile station 220's signal to interference ratio with respect to its desired signal from base station BS2.
To improve the signal-to-interference ratio for a given mobile station, the energy level of the signal transmitted to that mobile station is increased to appropriate levels. However, increasing the energy associated with one mobile station increases the interference associated with other mobile stations, e.g., those assigned to co-channels or adjacent channels. As such, radiocommunication systems must strike a balance between the desire to provide good reception at each mobile station individually and the desire to limit interference globally within the system. These competing desires led to the implementation of power control techniques in radiocommunication systems.
Power control can be considered at two stages of a connection between a mobile station and a base station: (1) setting initial transmit power(s) at which the mobile station and base station will begin transmissions on the traffic channel and (2) adjusting the initial transmit power(s) to arrive at optimum transmit power(s) given the manner in which the system operator decides to weight the criteria described above. The latter category has given rise to, for example, so-called open loop and closed loop power control techniques, which will be well known to those skilled in the art, but are not particularly relevant to this specification and, therefore, are not further described herein.
Conventionally, setting the initial transmit power level for transmissions by a base station on the downlink to a particular mobile station has been accomplished using one of two techniques. First, some systems simply use a fixed, predetermined initial transmit power which is used to begin information transmission on all traffic channels. However, these systems generally create excessive interference, since the fixed, predetermined initial transmit power is set to a relatively high level.
Accordingly, other systems employ a second technique which obtains measurements from the mobile station of transmissions made by the base station on a control channel (i.e., an overhead signalling channel) and uses these measurements to determine an appropriate initial transmit power for sending information to that mobile station on a traffic channel. An example of this latter technique for determining an initial transmit power is described in U.S. Pat. No. 5,487,180 to Ohtake.
Like the technique employing fixed, initial transmission power, there are also several problems associated with this second conventional technique. For example, if the base station (or system) uses the measurements made by the mobile station of its received signal strength to estimate a downlink pathloss associated with its transmissions as part of its initial transmit power calculation, this downlink pathloss estimate will contain errors that result in a less than optimal initial power setting. The base station may subtract the signal strength at which the mobile unit received its transmissions from the power at which it made those transmissions to arrive at an estimate for the downlink pathloss. However, both the received signal strength and the base station's own power may be measured relatively imprecisely, causing the downlink pathloss estimate to be erroneous and resulting in an improper initial transmit power setting. Moreover, the received signal strength measured by either or both of the mobile and base stations may include some component attributable to interference and/or noise. Additionally, some older systems (e.g., AMPS) do not even provide a capability for relaying mobile station measurements to the system, making downlink pathloss estimation impossible.
It is therefore an exemplary objective of the present invention to provide a technique and a system for more accurately estimating an initial transmit power to be used in transmitting to a mobile station.